Lithium ion batteries, nickel hydrogen batteries and other secondary batteries have recently gained importance as power sources for installation on vehicle or power sources for personal computers and portable terminals. In particular, lightweight lithium ion batteries that make it possible to obtain a high energy density are expected to be advantageously used as high-output power sources for installation on vehicles.
A typical configuration of a lithium ion battery includes an electrode of a configuration in which a material (electrode active material) that can reversibly absorb and desorb lithium ions is formed on an electrically conductive member (electrode collector). For example, an oxide including lithium and a transition metal element or transition metal elements of two or more kinds as constituent metal elements can be used as a positive electrode active material for use in a positive electrode. An elongated sheet-like member based on aluminum or an aluminum alloy can be used as the electrode collector (also referred to hereinbelow as “positive electrode collector”) for use in a positive electrode. Such a positive electrode for a battery is manufactured by dispersing a positive electrode active material in an adequate solvent (for example water) and kneading to prepare a positive electrode active material paste, coating a positive electrode collector with the paste and drying. Examples of the conventional techniques relating the manufacture of electrodes of such kind are described in Patent Documents 1 to 4.    Patent Document 1: Japanese Patent Application Laid-Open No. 2002-141059.    Patent Document 2: Japanese Patent Application Laid-Open No. H8-069791.    Patent Document 3: Japanese Patent Application Laid-Open No. 2008-226515.    Patent Document 4: Japanese Patent Application Laid-Open No. H11-204108.
A coating apparatus 200 such as shown in FIG. 5 is generally known as an apparatus for coating the electrode collector with the electrode active material paste. In the coating apparatus 200, as shown in FIG. 5, an elongated sheet-like electrode collector 210 is passed through a gap (coating gap d) between a backup roll 220 and a die 230, while conveying the collector by rotation of the backup roll 220, and an electrode active material paste 240 is applied from the die 230 on the electrode collector 210. A solvent (for example, water) contained in the electrode active material paste 240 is then evaporated in a drying furnace 250 and an elongated sheet-like electrode is manufactured.
Even among the equipment used to manufacture lithium ion batteries, the coating apparatus 200 is typically a high-cost unit. Accordingly, attempts have been made to reduce the cost by implementing the coating and drying operations of the electrode active material paste at a high speed and increasing the operation efficiency of the coating apparatus 200. For example, in order to increase the operation efficiency of the coating apparatus 200, it is preferred that the amount of solvent in the electrode active material paste 240 be reduced and solid fraction ratio be increased. Where the amount of solvent in the electrode active material paste 240 is reduced, the paste can be easily dried. Therefore, the drying time (time of passing through the drying furnace 250) can be reduced and the coating apparatus 200 can be operated with good efficiency.
However, where the amount of solvent in the electrode active material paste 240 is reduced, the paste density increases. Therefore, it is necessary to narrow the coating gap d and decrease the thickness of the coated film of the electrode active material paste 240. Where the coating gap d is narrowed, the electrode active material paste 240 cannot be smoothly separated from the die 230 and the behavior of the electrode active material paste 240 becomes unstable, thereby causing streaks and thickness unevenness on the coated surface.